When can a Kohen no longer become impure to bury his sister?

A Kohen is obligated to oversee the burial of 6 different blood relations; his parents, his son and daughter, and brother, and sister, who had "never had relations with a man." In addition, a Kohen may become impure to bury his wife. 

The salient question is what happens to the closeness between brother and sister once she loses her virginity. 

It would seem difficult to suggest that there's a wedge between the two of them as a result of her intercourse with a man; it could also be that as the years go on, and perhaps she and he have children of their own that they in fact feel greater closeness, their children – cousins – growing up together, with the sorrow the man feels upon one day losing his sister becoming even more pronounced. 

I'd like to suggest that the Torah view on burial was one of responsibility. A man is always responsible for his parents, and – we should not know such suffering – one's children. A brother is also under one's domain and as a responsible adult it is requisite that you see to the respect your brother deserves posthumously as well. But, what about one's sister - when she's become purportedly under another's domain? What happens when she's slept with someone else (the presumption, predominantly being that that happened in the course of marriage)? 

Another man has already taken her under her wing, taken responsibility for her, and is charged to seeing to her burial (were she to pass away first), and therefore, by default it is no longer under the brother's jurisdiction. 

The Torah's modus operandi was to see to a proper balance, and closeness between a Kohen, or priest, and his family. He couldn't become disengaged from his family but rather needed to see to their wellbeing, in life and death, but at the same time, the hope was to minimize his exposure to impurity, and therefore by default, once a man's sister has been with another man, she is no longer under his aegis vis a vis the responsibility that would be incumbent upon him. 

That approach, perhaps patriarchal and very anachronistic, is one that applies to other arenas of fraternal relationships. Males would inherit, to the exclusion of women, because they needed to see to their sisters' financial wellbeing; were there to be enough money, the boys needed to become paupers and beggars, writes the Gemara, with the girls provided for. Likewise, the sages teach that the verse, "Honor your father and your mother," pertains to your older brother as well. Shir Ha'Shirim, or the Song of Songs, in the allegory, refers to a beloved as a brother; the woman turns to her lover and states, "If you could only be like a brother to me."

That brotherhood, and responsibility, is likely much more mutual today, sisters looking after their brothers to the same extent as they look after them, but in understanding the laws of impurity and when a Kohen was allowed to become impure, it is most important as well to understand the germane historical context.